1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly, to a liquid ejection head having a liquid flow channel (nozzle flow channel) of which at least one wall surface is constituted by an elastic member.
2. Description of the Related Art
In the related art, an inkjet recording apparatus is known which includes a liquid ejection head (hereinafter referred simply to as “head”) having a plurality of nozzles, and which records images onto a recording medium by ejecting ink droplets from the nozzles toward the recording medium. One type of head is, for example, a piezoelectric type which pressurizes ink inside pressure chambers through the deformation of actuators, which are typically piezoelectric elements, thereby ejecting droplets of the ink from nozzles connected to the pressure chambers.
In recent years, there have been demands for recording images of high resolution and high definition at high speed, and hence the development of heads capable of ejecting very small droplets of high-viscosity inks having various additional functions has become necessary. However, if it is sought to eject very small droplets (for example, approximately 0.5 pl (picoliters) each) of ink having high-viscosity (for example, approximately 5 to 10 cP (centipoises)) with the piezoelectric type head in the related art, then various problems such as the following arise.
In a piezoelectric type head in the related art, a phenomenon (commonly called “trailing”) is liable to occur in which an ink droplet ejected from a nozzle forms a column shape trailing behind during its flight, and very small ink droplets (hereinafter referred to as “satellite droplets”) following the main ink droplet are thereby generated. The satellite droplets are deposited on the recording medium and lead to a decline in image quality. In order to prevent the generation of the satellite droplets, in Japanese Patent Application Publication No. 7-76087, for example, an ink droplet is ejected by switching the rates of drive voltage change of the actuator in two stages. However, in cases of increased viscosity of the ink, the ink column becomes longer and velocity distribution inside the ink column also becomes scattered, leading to the possible generation of a large number of satellite droplets. Moreover, if the ink droplet intendedly ejected from the nozzle is made very small in size, then the ink column itself becomes narrower, and satellite droplets become more liable to be generated, due to the effect of the Rayleigh-Taylor instability. Therefore, when ejecting a very small droplet of ink having high viscosity, it is difficult to prevent the occurrence of the trailing phenomenon, even if the actuator drive voltage is controlled in the method as disclosed in Japanese Patent Application Publication No. 7-76087.
In order to enable the ejection of high-viscosity ink, it is necessary to raise the power of the actuators, for instance, by increasing the number of layers of laminated piezoelectric body, or by raising the drive voltage; however, both manners have their limits on raising the power of the actuators. Moreover, there is also the problem of reduced ejection frequency due to delays in refilling.
In order to enable the ejection of very small droplets, it is also necessary to raise the drive frequency of the actuators. In a piezoelectric type head according to the related art, ejection is carried out by using what is known as full system resonance, whereby ink is ejected by controlling the drive frequency of the actuators in such a manner that the drive frequency substantially coincides with the resonance frequency of the ink inside the head (i.e., inside pressure chambers). Therefore, in order to increase the drive frequency of the actuators, it is necessary to increase the resonance frequency of the pressure chambers by reducing the size (volume) of the pressure chambers, and this leads to major restrictions on the design of the head.
As described above, in the piezoelectric type heads according to the related art, various problems arise when very small droplets of ink having high viscosity are sought to be ejected, and therefore it has been extremely difficult to achieve ejection of fine droplets of ink having high viscosity.